I really do not think this can be a software issue. I’ve tried many different kernels and different distros (with slightly different versions of ffmpeg/mpv as well as many different media). The exact same software on the same laptop works if I just set the cpu governor to performance.
Heck, from what I can understand linux is actually hardly involved with the variable cpu frequency once the governor is set… (and while I checked the driver, I see it really does ignore the max freq directive /sigh. intel pstate is complicated stuff.)
After disabling all drm modules / running mpv with null output from the framebuffer I really do not see what else I could try to convince you otherwise – I agree this likely can be solved by a microcode update (I guess it depends on how you define that, firmware is somewhere in between) but it’s not really something we can deal with directly and the feedback I have had on the intel ‘community forum’ is pretty disappointing so far.
Anyway, it would already be great if you can just get them to see if they can reproduce. It really shouldn’t take much time as mpv is packaged in PureOS, it’s really just a matter of installing a package, downloading a file, starting a couple of playbacks with default settings and wait 
The main thing I am curious about right now is whether all the librem 15v3 have the problem or if only a handful do, and if so maybe find a common factor (does the nvme drive consume too much power or something? I can’t really play with that until I get back from this trip)
Thanks!